1. Field of the Invention
The present invention relates to compositions and methods of fracturing hydrocarbon producing formations. More specifically, the present invention relates to a crosslinking system for use with the fracturing fluids to increase viscosity of the fracturing fluids.
2. Description of the Related Art
Hydraulic fracturing techniques are widely used to enhance oil and gas production from subterranean formations. During hydraulic fracturing, a fluid is injected into a well bore under high pressure. Once the natural reservoir fracture gradient is exceeded, the fracturing fluid initiates a fracture in the formation that generally continues to grow during pumping. The treatment design generally requires the fluid to reach a maximum viscosity as it enters the fracture that affects the fracture length and width. The viscosity of most fracturing fluids is generated from water-soluble polysaccharides, such as galactomannans or cellulose derivatives. Linear gels that can be operated at ambient temperature do not have the necessary viscosity for proper proppant transferring at elevated temperature. The use of crosslinking agents or crosslinkers, such as borate, titanate, or zirconium (Zr) ions, can further increase the viscosity. The gelled fluid can be accompanied by a propping agent (i.e., proppant) that results in placement of the proppant within the fracture that has been produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
Guar based fracturing fluids are the most commonly used fluids in reservoir stimulation. As indicated previously, stimulation of oil and gas wells has been improved by the ability to crosslink fracturing fluids, such as guar. Crosslinking agents are used to significantly improve the viscosity of the system for various downhole conditions. Some common crosslinking agents include boron and zirconium or other metallic compounds. Boron crosslinked gels are more commonly used due to its reversibility to mechanical shearing and favorable environmental properties.
While boron and zirconium crosslinking agents are effective for many types of guar based fracturing fluids, a certain amount of the guar polymer is needed to achieve the viscosity necessary to fractionate the formation. It is desirable to use as little polymer as possible in a fracturing fluid so that the overall cost of the fracturing job is lower, less polymer residue remains in the fracture and the sand pack after breaking, and formation damage is minimized.
In view of the foregoing, a need exists for a crosslinking agent that would effectively increase the viscosity of the polymer, which simultaneously reduces the polymer loading as much as possible in fracturing fluids. Additionally, it would be advantageous if such crosslinking system is compatible with existing fracturing systems.